1. Field of the Invention
The present invention relates to Ziegler-Natta (ZN) catalyzed ethylene-alpha olefin copolymers having densities of about 0.870 g/cc or higher, processes for making the same, and articles made of this new composition.
2. Description of the Related Art
Various types of polyethylene are known in the art. Low density polyethylene (LDPE) is generally prepared at high pressure using free radical initiators and typically has a density in the range of 0.9100-0.9400 g/cc. High density polyethylene (HDPE) usually has a density in the range of 0.9400 to 0.9600 g/cc, which is prepared with Ziegler-Natta type catalysts or single-site type catalysts (such as metallocene catalysts) at low or moderate pressures. HDPE is generally polymerized without comonomer, or alternatively with a small amount of comonomers with fewer short chain branches (SCB) than LLDPE. Linear low density polyethylene (LLDPE) is generally prepared in the same manner as HDPE, except it incorporates a relatively higher amount of alpha-olefin comonomers. By way of example, comonomers such as 1-butene, 1-hexene, or 1-octene are used to incorporate enough SCB into the otherwise linear polymers to depress the density of resultant polymers into the range of that of LDPE.
Conventional Ziegler-Natta catalyzed polyethylene copolymers such as LLDPE have both a relatively broad molecular weight distribution and a relatively broad comonomer distribution in which the comonomers are predominately incorporated into the low molecular weight polymer molecules or short polyethylene chains whereas the long polyethylene chains or high molecular weight polymer molecules do not contain a meaningful amount of comonomers. In other words, the conventional Ziegler-Natta catalyzed ethylene copolymers exhibit a heterogeneous SCB distribution among polymer chains of different molecular weight. This lack of compositional homogeneity is associated with several disadvantages including “organoleptic” problems caused by low molecular weight material and suboptimal impact strengths which are believed to be caused by the crystallinity of the homopolymer fraction.
Single-site catalysts normally produce resins with a narrow composition distribution in which comonomers are substantially uniformly distributed among the polymer chains of different molecular weight. As a result, both short chain branch distribution and polymer chain distribution of single-site catalyzed copolymers are known to be homogeneous.
It is well known that composition distribution affects the properties of copolymers. For example, extractable content, tear strength, dart impact, heat sealing strength, and environmental stress crack resistance (ESCR) can all be affected by composition distribution. Conventional Ziegler-Natta catalyzed LLDPE exhibiting a broad composition distribution and broad molecular weight distribution is known to have good processability as measured by extruder pressures and motor load. In film applications, conventional Ziegler-Natta catalyzed LLDPE (ZN LLDPE) exhibits good physical properties as related to tensile and tear strengths, but shows low dart drop impact strength. Single-site catalyzed LLDPE (mLLDPE), having a narrow composition distribution and narrow molecular weight distribution, is known to produce tough films with high dart impact and puncture properties. But the single-site catalyzed LLDPE exhibits adverse processability and weak film tensile properties (e.g. MD tear strength).
As such, it is highly desirable to attain polyethylene resins that exhibit ZN LLDPE type processability and a tear strength that is higher than or equivalent to ZN LLDPE, and a dart impact strength which is comparable to or better than that of mLLDPE. Theoretically, it is possible to improve the toughness of films (e.g. MD tensile strength) by increasing the amount of orientation in the machine direction during film fabrication. However, conventional knowledge in the polyethylene film art suggests that by increasing the machine direction (MD) orientation in films during manufacturing, other physical properties, such as MD tear strength, will significantly decrease.
Certain advantages were known in the prior art regarding super-hexene ZN LLDPE for enhancing toughness properties such as dart impact while maintaining the MD tear of conventional ZN LLDPE. The molecular weight distribution of super-hexene LLDPE is narrower than that of conventional ZN polymers but the composition distribution still resembles that conventional ZN LLDPE. As a result, the dart impact strength is still noticeably lower than that of single-site catalyst-based LLDPE.
Therefore, there is a need for a new LLDPE composition that would exhibit a balance of good processability and desirable physical properties. The resins of the present invention were found to match these requirements, exhibiting a MD tear strength that is higher than that of super-hexene ZN LLDPE and a dart impact strength which is comparable to or better than that of mLLDPE.